Trawls and other towed nets are widely used in commercial fishing operations for capturing sea life used for human consumption and the manufacture of products. These nets, however, are notoriously nonselective. In other words, any organisms in the path of the net that are large enough to be retained by the mesh of the net and that are not able or inclined to swim out of its path will be captured. Typically, trawl fishermen find that their catches contain not only the sizes and species of marine life that they are seeking, but also include other species and size categories that are undesirable for various conservation, fisheries management, economic, or other reasons. Because capturing certain species of marine life exposes fishermen to severe penalties from fisheries enforcement agencies, fishermen often have to take costly and troublesome measures in order to avoid these unwanted catches, such as moving out of the area, altering their fishing tactics or gear, or curtailing their operations altogether. Because hand or mechanical sorting after the catch often results in destruction or injury of many of the unwanted catch, there is a need for a means for sorting out the undesirable catch while retaining the desirable species in the net as the gear is fishing and before it is hauled back to the ship.
One attempt at achieving this objective involved varying the mesh sizes in the critical parts of the trawl, making one portion large enough to retain the target species while letting the others escape through the enlarged openings. The drawback to this method is that sorting by size does not give satisfactory results because the undesirable organisms are sometimes the same size as the target species or even larger. For instance, in the West Coast shrimp fisheries, the small mesh sizes required to retain shrimp also retain large quantities of unwanted fish such as smelt. Another drawback is that even large mesh openings will not prevent crustaceans from becoming entangled in the netting. This can be very costly in areas such as the Bering Sea sole fishery where the fishermen face harsh sanctions for capturing various species of crabs that are found in the most productive sole grounds.